Recording systems in magneto-optical recording media are roughly classified into optical modulation systems and magnetic field modulation systems. Magnetic field modulation systems enable easy overwriting and are suitable for mark edge recording. The magnetic field modulation systems, however, require magnetic heads, and the inductance of the heads must be reduced for switching a large current flow when the magnetic field is switched at a high speed. In order to increase the recording magnetic field as much as possible by bringing the magnetic head as close as possible to the recording film of a recording medium, use of a floating magnetic head is effective in practice as described in Japanese Patent Laid-Open No. 63-217548.
In any case, it is preferable that magneto-optical recording media for magnetic field modulation systems are capable of recording at a low magnetic field. The reduction of the recording magnetic field is achieved by optimizing the heavy rare earth metal content in a ternary recording film composed of rare earth elements, Fe and Co, as described in page 53 of "The Paper of Technical Meeting on Magnetics MAG-86-95"; a fourth element such as Nd is added to enhance magnetic characteristics at low magnetic fields as described in page 268 of "Digests of the p11th Annual Conference on Magnetics in Japan"; a response to external magnetic fields is improved by reducing a stray field by exchange coupling of magnetic fields with different compositions as described in Japanese Patent Laid-Open No. 62-128040; and a vertically magnetized film is stacked with a planar magnetic film to effectively collect a magnetic flux to the vertically magnetized film and thus to reduce the magnetic modulation field as described in Japanese Patent Laid-Open No. 61-188758.
It is, however, difficult to reduce the magnetic modulation field to less than .+-.100 Oe by these prior art technologies, and addition of the fourth element and stacking with the planar magnetic film will deteriorate readout characteristics. The present inventors have discovered that the magnetic field for modulation can be reduced by forming an ultra-thin magnetic layer adjoining the recording layer, wherein the Curie point of the magnetic field is higher than that of the recording layer, and have applied for a patent (Japanese Patent Laid-Open No. 6-309711). Also, the present inventors have discovered that the magnetic modulation field is further reduced by a combination of a recording layer and an ultra-thin magnetic layer, in which the heavy rare earth metal contents in the recording layer are reduced. Although the recording magnetic modulation field has been considerably reduced by these inventions, a further reduction is required.
Accordingly, an object of the present invention is to provide a magneto-optical recording medium for magnetic field modulation systems, capable of satisfactory recording by a smaller magnetic modulation field than that of conventional technologies.